Group 9 pneumococci is one of the most common groups causing pneumococcal diseases. Types 9N and 9V polysaccharides are contained in the current 23-valent pneumococcal vaccine. Type 9V is infectious mainly in young children, and is poorly immunogenic in this population. Thus, we have studied the immune response of neonate mice to 9V PS using polysaccharide-protein conjugates. Type 9V PS was conjugated to tetanus toxoid (TT) by the method of cyanogen bromide activation of PS and the coupling reagent, carbodiimide, for conjugation. The ratio of protein/polysaccharide in the conjugate ranged between 0.23-0.41. The optimum doses for antibody responses were 2.0 mg (IgM Ab) for 9V PS; 1-5 mg (IgM Ab) and 5.0-25 mg (IgG Ab) for 9V PS-TT conjugate. Type 9V PS as well as 9V PS-TT conjugate induced maximum IgM antibody response in mice 7-9 days after injection; IgG Ab response was too low to be detected in both groups. However, when the booster dose was given at 14 days after primary injection, a high level of IgG titer (1.78-2.33 mg/ml serum) was induced in mice given the conjugate immunogen; the IgG Ab response was not detectable in mice given 9V PS. The IgM Ab response showed 3-4 fold increase after the booster injection with the conjugate. In contrast, no increase in IgM Ab response was observed in mice given 9V PS booster injection. Studies on effects of the immunization of female mice with PS-protein conjugate before mating or during pregnancy on the antibody responses of neonates to 9V PS are in progress.